1. Field of the Invention:
This invention relates to an improved method for the preparation of naphthalene dicarboxylic acids in which a diisopropylnaphthalene is oxidized with molecular oxygen in a solvent in the presence of an oxidation catalyst. This invention is also concerned with a method for recovering a cerium compound used as a catalytic component from a crude naphthalene dicarboxylic acid product.
Naphthalene dicarboxylic acids including 2,6- and 2,7-naphthalene dicarboxylic acid are useful as starting materials for various high-performance polymers such as polyethylene naphthalates and other polyesters, and polyamides which, in turn, are useful for manufacturing films and fibers having outstanding heat resistance and mechanical properties.
2. Prior Art:
Naphthalene dicarboxylic acids (hereinunder referred to as NDCA) such as 2,6-NDCA and 2,7-NDCA are generally prepared by oxidizing a corresponding dialkyl-naphthalene with molecular oxygen in a lower fatty acid such as acetic acid as a solvent in the presence of a catalyst system comprising bromine and at least one heavy metal selected from cobalt and manganese.
When 2,6-dimethylnaphthalene is used as a starting material for oxidation, 2,6-NDCA can be obtained with a relatively high yield. However, the synthesis of the starting 2,6-dimethylnaphthalene which may be performed by various methods including methylation of naphthalene, isomerization of other dimethyl naphthalene isomers, and disproportionation of monoethylnaphthalene produces the desired 2,6-dimethyl isomer with a very low yield and the product always contains considerble amounts of other isomer. There are ten isomers of dimethylnaphthalene having similar physical properties, and it is rather difficult to separate the 2,6-isomer from a mixture of different isomers. Therefore, the preparation of 2,6-NDCA from 2,6-dimethylnaphthalene is not economical.
On the other hand, diisopropylnaphthalenes (hereinbelow referred to as DIPN) can be easily synthesized, and it is relatively easy to separate the individual isomers thereof, e.g., the 2,6-isomer and the 2,7-isomer from a mixture of various isomers. However, the use of DIPN as a starting material to prepare NDCA by oxidation has the disadvantage that the yield of the oxidation product, NDCA, is much lower than when a dimethylnaphthalene is employed as a starting material.
There have been many proposals of methods for increasing the yield when preparing 2,6-NDCA from 2,6-DIPN by oxidation with molecular oxygen in the presence of a catalyst system comprising bromine and at least one heavy metal selected from cobalt and manganese.
These include a method in which at least 0.2 moles of the heavy metal are used for each mole of 2,6-DIPN or an intermediate thereof (Japanese Published Unexamined patent application No. 60-89445); a method in which the heavy metal is present in an amount of at least 1% based on the weight of the solvent (Japanese Published Unexamined patent application No. 60-89446); a method in which the catalyst system further contains an alkali metal (Japanese Published Unexamined patent application No. 61-246143); and a method in which bromine is present in an amount of 0.1%-10% based on the weight of the solvent (Japanese Published Unexamined patent application No. 61-246144).
It has also been proposed that a catalyst system comprising bromine and the heavy metals cobalt and cerium or nickel be used in order to increase the yield of 2,6-NDCA (Japanese Published Unexamined patent applications Nos. 62-212344 and 62-212343).
It is also important in the commercial production of NDCA by the above-mentioned catalytic oxidation to recover the expensive heavy metal catalytic components for reuse in order to reduce production costs.
With respect to a similar reaction system in which terephthalic acid is prepared by liquid-phase oxidation of p-xylene in the presence of a Br-Co-Mn catalyst system, it has been proposed that after the resulting terephthalic acid is collected by filtration, the cobalt and manganese be recovered as carbonates from the filtrate following evaporation of the solvent (Japanese patent Publication No. 46-14399 and Japanese Published Unexamined application No. 47-34088). Japanese Published Unexamined application No. 49-106986 discloses that the recovered cobalt and manganese components can be subjected to oxidation with molecular oxygen to remove the undesirable by-products deposited thereon and to enhance the activity of the recovered catalytic components.
With respect to the catalytic oxidation of 2,6-DIPN into 2,6-NDCA, Japanese Published Unexamined patent application No. 62-212345 discloses a process in which the reaction mixture is withdrawn from a reactor, crude 2,6-NDCA crystals precipitated are separated, and then washed with an aqueous solution of a mineral acid to dissolve and remove the heavy metal catalytic components (Co and Mn). The crystals are then subjected to salting-out with an alkali solution to precipitate dialkali salt of 2,6-NDCA, while the acidic washings are treated with an alkali carbonate to recover the heavy metal catalytic components as carbonates.
However, the recovery of other metallic components such as a cerium compound used in the catalyst from the reaction mixture of a catalytic oxidation of DIPN is not taught in the prior art.